1. Field of the Invention
The present invention relates to; a chemically amplified resist composition that is sensitive to a high energy beam such as an ultraviolet beam, a deep-ultraviolet beam, an electron beam, an EUV (extreme ultraviolet beam), an X-ray, a γ-beam, and a synchrotron radiation beam and is usable for processing of a semiconductor, a photomask blanks, and so on; especially a sulfonium salt usable to prepare a chemically amplified resist composition, especially a chemically amplified positive resist composition, that is used in a photo-exposure step by a beam irradiation of a high energy beam including an electron beam and a deep-ultraviolet beam; a polymer; a chemically amplified resist composition, especially a chemically amplified positive resist composition, using the said polymer; and a resist patterning process using the same.
2. Description of the Related Art
In recent years, as an integrated circuit progresses toward further higher integration, a further finer pattern profile is required; and thus, a chemically amplified resist using an acid as a catalyst is mainly used in the patterning process of the size of 0.2 μm or less. In this method, a high energy beam, such as an ultraviolet beam, a deep-ultraviolet beam, and an electron beam, is used as a light source for it; and especially an electron beam lithography used as an ultrafine processing technology has become indispensable also as a processing method of a photomask blank used for producing a photomask for a semiconductor manufacturing.
Generally, an electron beam drawing is carried out by an electron beam with a method wherein, in the case of a positive type, an electron beam with a fine area irradiates progressively, without using a mask, on an area other than those areas where a resist film is intended to be remained. In this method, an entire area of a processing surface that is finely divided is scanned, whereby taking more time as compared with a one-time exposure using a photomask; and thus, a high sensitivity resist film is required so that the throughput thereof may not be impaired. In addition, because the drawing takes a long time, difference between the parts drawn in earlier time and in later time tends to occur easily; and thus, temporary stability of an exposed part under vacuum is an important functional requirement item. In processing of a photomask blanks, which is an especially important application thereof, there is a case that a surface material affects a pattern profile of a chemically amplified resist easily, such as, for example, a film of a chromium compound including a chromium oxide formed on a photomask substrate; and thus, to keep rectangularity of a pattern profile of a resist film regardless of kinds of the substrate is also one important function for a high resolution and to keep a profile after etching.
Meanwhile, many improvements have been made in resist sensitivity and control of a pattern profile as mentioned above by selecting or combining materials used in a resist composition, process conditions, and so on. One item of such improvements relates to the issue of acid diffusion that has substantial influence on resolution of a chemically amplified resist. In a photomask processing, a resist pattern profile obtained as mentioned above needs to be unchanged regardless of the time passage after exposure to post-exposure baking; and here, a major cause for this change with the time passage is diffusion of an acid generated by the photo-exposure. Many studies have been made on the issue of the acid diffusion because this influences sensitivity and resolution not only in a photo mask-processing but also in a general resist composition.
Especially a method wherein the diffusion is suppressed by bonding a sulfonic acid generated by photo-exposure to a resin used in a resist composition, as disclosed in Japanese Patent Laid-Open (kokai) No. H09-325497, receives an attention as a suppression method that is based on a mechanism different from the suppression method using a basic compound. As the need for a further finer pattern increases, many attempts to improve this method have been made; the method described in Japanese Patent Laid-Open (kokai) 2008-133448 is an example of an attempt to improve an acid strength.
Meanwhile, a polymer containing many aromatic skeletons having an acidic side chain, for example, polyhydroxy styrene, has been used effectively as a resist material for a KrF excimer laser; but it has not been used as a resist material for an ArF excimer laser because it has a large absorption in wavelength of near 200 nm of the light. Nevertheless, in view of its high etching resistance, it is an important resist material for an electron beam and an EUV (extreme ultraviolet), which are useful technologies to form a smaller pattern than a processing limit by an ArF excimer laser.
As a base polymer for a positive type electron beam resist composition and EUV resist composition, a material whose acid-labile protective group that protects an acidic functional group of a phenolic side chain contained in the base polymer is deprotected by an acid catalyst generated from a photo acid generator by exposure thereof to a high energy beam thereby being made soluble in an alkaline developer is mainly used. As the acid-labile protective group mentioned above, a tertiary alkyl group, a t-butoxycarbonyl group, an acetal group, and the like have been mainly used. Here, when a protective group having a comparatively small activation energy necessary for deprotection, such as for example, an acetal group, is used, even though there is a merit of obtaining a highly sensitive resist film, the deprotection reaction takes place even in an unexposed part of the resist film if suppression of the generated acid-diffusion is insufficient, thereby causing a problem of deterioration of line edge roughness (LER) and decrease in in-plane uniformity of the pattern line width (that is, CDU (Critical Dimension Uniformity)).
The method for suppressing acid diffusion by bonding a repeating unit generating an acid by photo-exposure to a base polymer as shown in the foregoing Japanese Patent Laid-Open (kokai) Nos. H09-325497 and 2008-133448 is effective to obtain a pattern having a small LER. However, depending on the structure of the repeating unit mentioned above, there has been a case to cause a problem of poor solubility of the base polymer having the repeating unit to generate an acid by photo-exposure in a solvent. There has also been a problem of forming a pattern with a large LER when a resin has a repeating unit to generate an acid of high pKa such as a fluorinated alkanesulfonic acid and a repeating unit having an acetal group. That is, this is because the acetal group, which has relatively low activation energy for deprotection, is deprotected due to too high acidity of a minute amount of the fluorinated alkanesulfonic acid diffused to an unexposed part, even if acid diffusion is suppressed. In addition, there have been problems of easily causing pattern peel-off and pattern fall because a polymer containing a large amount of fluorine impairs adhesion with a substrate.